The principal goal is to design and test an automatic drug administration system for lowering the mean arterial blood pressure in dogs and maintaining it at a desired level. It will be accomplished by designing an (approximately optimal) controller whose input is the measured mean arterial blood pressure and output is fed into a syringe pump. The controller will be implemented by means of a small computer such as a micro-processor. The syringe pump controls the infusion rate of a hypotensive drug (e.g., nitroprusside) so that the mean arterial blood pressure will stay at the desired level. The automated drug administration system, which has a microprocessor in the feedback loop, will be tested on normo-, and hypertensive dogs using nitroprusside as the hypotensive agent. The hypertension in dogs will be produced by a constant infusion of norepinephrine. The system is first designed under the assumption that the mean arterial blood pressure is measured continuously in time. The feedback system will then be refined by assuming that the mean arterial blood pressure measurements are available only intermittently, as for example, from a cuff. Thus, there is no need for a cannula for blood pressure measurements. This drug delivery system will be designed, implemented and tested on hypertensive dogs. The range of the mean arterial blood pressure over which the device can be used will be determined. Additional limitations on the operation of the system such as the maximal change in the mean arterial blood pressure which can safely be performed will be specified.